This invention relates to an improved process for the preparation of dimethyl terephthalate. More particularly, it relates to a catalyst system of higher selectivity for the air oxidation of p-xylene/p-toluic acid methyl ester mixtures, for example, in accordance with German Pat. Nos. 949,564 and 1,041,945.
Dimethyl terephthalate is produced in numerous large-scale industrial plants in accordance with the "Katzschmann process", described in said patents. The principle of this process resides in that the p-toluic acid, which first evolves during the catalytic air oxidation of p-xylene in the liquid phase, is esterified with methanol, and the resultant methyl p-toluate is again recycled into the oxidation stage and is oxidized to monomethyl terephthalate. In practice, the process is conducted in such a manner that methyl p-toluate and p-xylene are oxidized with air together with cobalt salts soluble in the reaction medium, in a batch-wise or continuous fashion, and the thus-obtained acids are esterified with methanol. The mixture, consisting mainly of dimethyl terephthalate and methyl p-toluate, is then worked up, preferably by distillation, and the thus-obtained methyl p-toluate is recycled to the oxidation stage.
Numerous catalysts have been described for the air oxidation of alkyl aromatics, especially of p-xylene. These catalysts make it possible to effect, by increased activity, the direct oxidation up to terephthalic acid during the air oxidation of p-xylene conducted in the present of aliphatic lower monocarboxylic acids, which normally leads only to p-toluic acid. The increased activity is attained, for example, by adding Br.sup.- to cobalt- and manganese-containing catalysts. In the Katzschmann process for the production of dimethyl terephthalate, an increased activity of the catalyst is of minor importance, since the oxidation of p-xylene proceeds via the intermediate stage of methyl p-toluate with a sufficient speed to monomethyl terephthalate. In this connection, it is rather the selectivity of the catalyst which is of interest, i.e., the yield of dimethyl terephthalate. No catalyst systems have been known or reported heretofore which result in a higher selectivity or improved yields of dimethyl terephthalate in connection with the Katzschmann process, as compared to the generally employed cobalt catalyst. In addition to cobalt salts, salts of a great variety of metals have been examined with respect to their catalytic effect for conducting this process, for example, manganese, chromium, cerium and lead salts. These salts, however, exhibit a less favorable effectiveness [Katzschmann, "A Process for the Oxidation of Alkyl Aromatics", Chemie-Ingenieur-Technik (Chemical Engineering Technology), Vol. 38, 1966, pp. 1-10].
When juding the selectivity of an oxidation catalyst, consideration can be based on the fact that the main reaction in the reaction fundamental to the Katzschmann process, i.e., the oxidation of the alkyl groups of xylene and methyl p-toluate, can be controlled, observed or monitored approximately by the acid number of the reaction mixture, whereas the secondary reactions are indicated by the formation of CO.sub.2, CO, formic acid, acetic acid and higher molecular weight condensation products. Other yield determinations do not result in accurate data in experimental oxidations, because of the complicated nature of the composition of the reaction products.
Accordingly, one of the objects of the present invention is to provide a process for the preparation of dimethyl terephthalate which gives improved results over the prior art.
Another object of the present invention is to provide a catalyst system of high selectivity for obtaining high yields of dimethyl terephthalate.
A further object of the invention is to provide a process for the preparation of dimethyl terephthalate which may be carried out advantageously and efficaciously on an industrial scale to give a high yield of product.
These and other objects and advantages of the present invention will become apparent to those skilled in the art from a consideration of the following specification and claims.